Use dedicated disassembler instead of custom parser to fix "some instructions unknown" errors.

I noticed that mach_override failed to instrument a few functions in the Tcl library, yielding a 'some instructions unknown!' message. Instead of extending the simplistic assembler instruction recognition even further, I decided to resolve this by throwing the custom code away and instead use a proper disassembler to recognize the statements.

I chose to use http://udis86.sourceforge.net/ which I used successfully in the past - it's also very lightweight, just a bunch of C files with no external dependencies (note how minimal the modification to the Rakefile is).